1. Field of the Invention
The present invention generally relates to a balun, and more particularly to a Marchand balun that has a higher coupling coefficient and wider operation bandwidth. The microwave monolithic integrated circuit (MMIC) mixer based on the present invention can provide compact size compared to conventional ones and can be applied to more different types of systems.
2. The Prior Arts
A conventional balun is used between balanced transmission line and unbalanced transmission line to convert unbalanced signal to balanced signal, or balanced signal to unbalanced signal. Signal input into a balun from the unbalanced port would be converted by the balun to produce signals at the balanced ports of equal amplitude and 180 degree phase shift. A balun plays an important role in microwave and millimeter-wave systems, and can be applied to balanced amplifiers, mixers, voltage-controlled oscillators, phase shifters, and antennas, etc.
A balun can be made in different types of structure, such as active type, lumped component type, Marchand type, and Rat-race type structure. An active type balun has advantages of wide bandwidth and gain, but at the cost of larger noise and power dissipation. A lumped component balun has compact size by adopting lumped capacitors and lumped inductors but has smaller operation bandwidth. Therefore a lumped component balun is normally used in systems whose operation bandwidth is less than 10 GHz. On the other hand, a Rat-race balun contains three quarter-wave length transmission lines and one transmission ¾ wavelength line, while a Marchand balun contains two quarter-wave length coupled lines. The Marchand balun therefore has a more compact balun size.
The Marchand balun is extensively applied in microwave and millimeter-wave systems because of its wide operation bandwidth. Please refer to FIG. 1, which is a schematic view of a conventional Marchand balun.
As shown in FIG. 1, a Marchand balun 100 comprises a first edge-coupled-line set 102 containing two quarter-wave length coupled lines, and a second edge-coupled-line set 104 containing another two quarter-wave length coupled lines (R. Mongia, I. Bahl, and P. Bhartia, RF and Microwave Coupled-Line Circuits, Norwood, Mass.: Artech House, 1999, ch 11). When a signal at central frequency excites the balun 100 at the first port 110, balanced signals of equal amplitude and 180 degree phase shift are produced at the second and third ports 106 and 108. Due to its edge coupling, the Marchand balun 100 as shown in FIG. 1 would have leakage field and therefore result in less efficient electric field utilization.
FIG. 2 is a schematic view of another conventional Marchand balun disclosed by Basu. As shown in FIG. 2, the Marchand balun uses three edge-coupled lines and has wider operation bandwidth.
The Marchand balun 200 as shown in FIG. 2 contains two edge-coupled-line sets 204 and air bridges 202 (S. Basu, and S. A. Maas, “Design and performance of a planar star mixer,” IEEE Trans. on Microwave Theory and Techniques, vol. 41, pp. 2028-2030, November 1993.). Each edge-coupled-line set 204 contains three coupled lines. Air bridges 202 are connected between two coupled lines in the edge-coupled-line sets 204 and therefore these two coupled lines have equal potential. The air bridges 202 are provided at the ends of the connected coupled lines, and the ratio of the total width of the air bridges 202 to the total length of coupled lines is no more than 10%.
FIG. 3 is a schematic view of another conventional Marchand balun disclosed by Kamozaki (T. N. Trinh, W. S. Wong, D. Li, and J. R. Kessler, “Ion implanted W-band monolithic balanced mixers for broadband applications,” Microwave and Millimeter-Wave Monolithic Circuits, vol. 87, pp. 89-92, June 1987.). As shown in FIG. 3, the Marchand balun 300 contains two stacking coupled lines and is used for a 50-100 GHz mixer. The overlapping coupled lines of FIG. 3 and the three coupled lines of FIG. 2 are also of ¼ wavelength in length and therefore do not increase the size of Marchand balun compared to conventional Marchand baluns.